1. Field of the Invention
The invention relates to a device for the double-sided processing of flat workpieces, comprising upper and lower working discs, at least one of the discs being driven in rotary fashion by means of a drive, and which between themselves form a working gap in which is arranged a carrier disc with a cutout for at least one workpiece to be processed, wherein the carrier disc has teeth on its circumference by means of which it rolls on an inner and an outer gear wheel or pin ring if at least one of the gear wheels or pin rings is set in rotation, wherein the gear wheels or pin rings each have a multiplicity of gear or pin arrangements which the teeth of the carrier discs engage during rolling.
2. Background Art
With devices of this type, flat workpieces, for example semiconductor wafers, can be subjected to material removal processing, for example honing, lapping, polishing or grinding. For this purpose, the workpieces are held in floating fashion in cutouts in carrier discs guided in rotary fashion in the working gap and are simultaneously processed on both sides. In this case, the workpieces describe a cycloid movement in the working gap. With such devices, flat workpieces can be processed on both sides in a highly precise manner.
The contact between the outer teeth of the carrier discs and the teeth of the gear wheels or the pins of the pin rings results in wear on the gears or pins. It is therefore known from DE 295 20 741 U1 to mount sleeves in rotatable fashion on the pins of pin rings, wherein the carrier discs come into engagement with the sleeves. In the case of an embodiment of this type, frictional stress no longer occurs between the carrier disc teeth and the pins. Rather, such contact takes place between the sleeve and the pin. However, since the sleeve bears on the pin over a much greater length, the surface loading and hence the possible abrasion are correspondingly lower. Furthermore, the sleeves can be replaced in a simple manner in the event of wear. By contrast, replacing the pins is comparatively complicated. Further configurations of such sleeves have been disclosed by DE 101 59 848 B1 and DE 102 18 483 B4. EP 0 787 562 B1 discloses sleeves composed of a plastic material.
In the case of the known devices, one problem is that the loading of the carrier discs on account of contact with the gears or pins and sleeves can lead to the teeth of the carrier disc being bent away upward or downward, which regularly leads to damage to the workpieces and also the working discs or their working layers. On account of the low strength, this is particularly critical in the case of plastic carrier discs that are otherwise desired. Moreover, premature wear of the carrier discs can occur in the case of the known devices. This is because the carrier disc partially leaves the working gap, in particular in the region of the gear wheels or pin rings, which can perform undesirable vertical movements owing to the lack of guidance there by the working gap. When this part of the carrier disc reenters the working gap, these movements lead to undesirable contact between the carrier disc surface and the edge of the working discs or their working layer, as a result of which intensified wearing of the carrier disc surface occurs.
The present invention also relates to a method for the simultaneous double-sided material removal processing of a plurality of semiconductor wafers, in which each semiconductor wafer lies freely mobile in a recess of one of a plurality of carrier discs set in rotation by means of an annular outer wheel or pin ring and an annular inner gear wheel or pin ring and thereby moves on a cycloid path curve, while the semiconductor wafers are processed so as to remove material between two rotating annular working discs, and the carrier discs and/or semiconductor wafers temporarily leave the working gap, delimited by the working discs, with a part of their surface during the processing.
For electronics, microelectronics and micro-electromechanics, semiconductor wafers with extreme requirements for global and local planarity, one-side referenced local planarity (nanotopology), roughness and cleanness are used as starting materials (substrates). Semiconductor wafers are wafers of semiconductor materials, in particular compound semiconductors such as gallium arsenide or elementary semiconductors such as silicon and germanium.
According to the prior art, semiconductor wafers are produced in a multiplicity of successive process steps. In general, the following production sequence is used:    production of a monocrystalline semiconductor rod (crystal growth),    cutting the rod into individual wafers (internal hole or wire sawing),    mechanical wafer preparation (lapping, grinding),    chemical wafer preparation (alkaline or acid etching)    chemo-mechanical wafer preparation: double-sided polishing (DSP)=stock polishing, single-sided haze-free or mirror polishing with a soft polishing pad (CMP)    optionally further processing or coating steps (for example epitaxy, annealing)
Mechanical processing of the semiconductor wafers serves primarily for global planarization of the semiconductor wafer, and also for thickness calibration of the semiconductor wafers as well as removal of the crystalline-damaged surface layer and of processing traces (sawing cuts, incision marks) caused by the previous cutting process.
Methods known in the prior art for mechanical wafer preparation are single-sided grinding (SSG) with a cup grinding disc which contains a bound grinding agent, simultaneous grinding of both sides of the semiconductor wafer together between two cup grinding discs (“double-disc grinding”, DDG) and lapping both sides of a plurality of semiconductor wafers simultaneously between two annular working discs while supplying a slurry of free grinding agent (double-sided plane-parallel lapping, “lapping”).
DE 103 44 602 A1 and DE 10 2006 032 455 A1 disclose methods for simultaneously grinding both sides of a plurality of semiconductor wafers together with a movement process similar to that of lapping, but characterized by the use of a grinding agent which is bound firmly in working layers (“films”, “pads”) which are applied onto the working discs. Such a method is referred to as “fine grinding with lapping kinematics” or “planetary pad grinding” (PPG).
Working layers used in PPG, which are adhesively bonded onto the two working discs, are described for example in U.S. Pat. No. 6,007,407 A and U.S. Pat. No. 6,599,177 B2. During the processing, the semiconductor wafers are placed into thin guide cages, so-called carrier discs, which have corresponding openings for receiving the semiconductor wafers. The carrier discs have outer teeth, which engage in a rolling device comprising an inner and outer gear wheel and are moved thereby in the working gap formed between the upper and lower working discs.
The ability to carry out the PPG method is crucially determined by the properties of the carrier discs and their guiding during the rolling movement. The semiconductor wafers must temporarily leave the working gap with a part of their surface during the processing. This temporary projection of a part of the area of the workpieces from the working gap will be referred to as the “workpiece excursion”. The latter ensures that all regions of the tool are used uniformly and experience uniform wear which preserves their shape, and the desired plane-parallel shape is imparted to the semiconductor wafers without “balling” (thickness reduction toward the margin of the semiconductor wafer). The same applies similarly for lapping with a free lapping abrasive.
The methods of PPG grinding known in the prior art, for example as described in DE 103 44 602 A1 and DE 10 2006 032 455 A1, are however disadvantageous in this regard. With the methods known from the prior art, it is not possible to provide semiconductor wafers with sufficient planarity as far as the outermost marginal region, which are suitable for particularly demanding applications and future technology generations.
Specifically, it has been found that the carrier discs are susceptible to vertical dislocation from their central position until they disengage from the rolling device owing to strong bending. This is to be expected in particular when high or strongly alternating process forces act on the carrier discs as in the case of high removal rates, unfavorably selected process kinematics, or when using particularly fine abrasives in the grinding pad.
The dislocation of the carrier discs is promoted because they have only a small total thickness (at most slightly greater than the final thickness of the semiconductor wafers to be processed) and thus have only a limited strength against bending. Furthermore, the carrier disc is conventionally made of a steel core which is provided with a protective layer. Direct contact of the steel core and the abrasive preferably used in PPG, i.e. diamond, leads to wear of the microedges of the diamond grains owing to the high solubility of carbon in iron, and therefore rapid loss of the cutting acuity of the working layers being used.
Frequent sharpening associated with high wear of the working layers, with the concomitant unstable process management, would also compromise the properties of the semiconductor wafers thereby processed and therefore make the use of PPG methods not only uneconomical, but possibly even unviable for future technology generations.
As is known, the protective layers applied onto the steel core of the carrier disc experience wear. They should therefore have a usable thickness which is as large as possible, in order to allow economical lifetimes of the consumable constituted by the “carrier disc”. The protective layers are furthermore required in order to achieve low sliding friction between the working layers and the carrier discs. Suitable layers consist, for example, of polyurethane. The layer is conventionally soft and does not therefore contribute to the stiffness of the carrier disc. The remaining steel core is therefore much thinner than the target thickness of the semiconductor wafers after the processing by means of PPG.
If the target thickness of a semiconductor wafer with a diameter of 300 mm after processing by means of PPG is for example 825 μm and the total thickness of the carrier disc being used is 800 μm, then 500-600 μm of this 800 μm total thickness of the carrier disc is given over to the steel core which imparts stiffness, and 100-150 μm each to the anti-wear coating on the two sides.
If for comparison the target thickness of the semiconductor wafer after processing by means of lapping is likewise 825 μm, then the carrier disc used for the lapping consists entirely of stiffness-imparting steel and has a thickness of 800 μm.
Since the bending of a plate, for the same material and the same shape and design, is known to vary with the third power of its thickness, a carrier disc with a 500 μm thick steel core bends during PPG about four times as much as an 800 μm thick carrier disc during lapping.
For a carrier disc with a 600 μm thick steel core, the bending during PPG is still 2.4 times that of an 800 μm thick carrier disc during lapping.
In the working gap, the maximum deviation from the plane setting of the carrier disc is limited by the difference between the carrier disc thickness and instantaneous thickness of the semiconductor wafers. This is typically at most 100 μm. Wherever the carrier disc projects inward and outward from the annular working gap and engages into the rolling device comprising the inner and outer pin ring, no measures are implemented in the prior art of PPG methods to limit the possible bending of the carrier disc. Owing to the workpiece excursion required, this unguided region is particularly large.
Bending of the carrier discs leads to the following disadvantages for the semiconductor wafers and for the carrier discs, and therefore to an unstable and critical overall process:    a) The semiconductor wafer always extends partially from the reception opening of the carrier disc and is forced back in when it re-enters the working gap. This also bends the semiconductor wafer and presses it onto the outer or inner edge of the grinding pad. This can lead to the formation of local scratches and geometrical defects in the marginal region owing to the increased grinding effect.    b) The continual insertion and extraction of the semiconductor wafer into and from the bent carrier disc roughens the reception opening of the carrier disc, which is generally lined with an insert made of a soft plastic. Sometimes, the lining of the reception opening may even be torn out of the carrier disc. In any event, the service life of the carrier discs being used suffers detrimentally.    c) The roughened lining of the reception opening of the carrier disc brakes or stops the desired free rotation of the semiconductor wafers inside the reception opening. This can lead to planarity defects of the semiconductor wafer in respect of global planarity (for example TTV=total thickness variance) and local planarity (nanotopography).    d) The carrier disc, bent in its excursion, exerts high forces on the grinding bodies when it re-enters the working gap, in particular on the outer and inner edges of the annular working layer. The working layer can thereby be damaged. Entire grinding bodies (“tiles”) can be torn out, or at least parts thereof can be displaced. If these fragments enter between the semiconductor wafer and the working layer, fracture of the semiconductor wafer is possible owing to the high point loading.    e) The bending of the carrier disc, with increased point loading of its protective layer at the points which sweep over the edge of the working layer, leads to greatly increased local wear. This considerably limits the lifetime of the carrier disc and makes the method uneconomical. The increased wear of the protective layer furthermore makes the working layer blunt. This necessitates frequent resharpening processes which consume time and material, and are therefore detrimental to the economic viability of the method. Furthermore, the frequent process interruptions have a negative effect on the properties of the semiconductor wafers thus processed.
JP 11254303 A2 discloses a device for guiding the carrier discs, which consists of two upper and lower spacers which converge conically or in a wedge shape and which are arranged on the inner margin of an outer gear wheel of the lapping machine. The deformation of thin carrier discs is intended to be able to be prevented by means of such a device. However, the modification described therein for the lapping machine, which moreover is directed at the processing of glass substrates, has substantial disadvantages and is unsuitable for carrying out methods of lapping and PPG grinding with workpiece excursion.
Both when lapping with free cutting abrasive in a slurry and for PPG grinding with abrasive bound firmly in grinding pads, the working layers (cast-metal lapping plates or grinding pad) experience constant wear. The height of the lapping plate or grinding pad decreases continuously and the position of the plane, in which the carrier discs move in the working gap formed between the lapping plates or grinding pads, is displaced progressively.
With increasing wear of the working layer and displacement of the movement plane of the carrier discs, the forcible guiding device disclosed in JP 11254303 A2 constrains the toothed outer region of the carrier discs to roll increasingly in a different plane. This means that the wedge-shaped guide blocks, screwed firmly to the outer toothed wheel, would additionally bend the carrier disc with increasing wear of the working disc, which is disadvantageous. Another disadvantage is that the guide blocks need to be unscrewed before it is possible to change the carrier disc, which is necessary from time to time. This represents additional outlay.
In PPG grinding methods, carrier discs are conventionally used with a coating, which is necessary in order to avoid direct contact between the stiffness-imparting core of the carrier disc and the abrasive of the grinding pad (for example diamond). Owing to the design, the spacers described in JP 11254303 A2 engage far into the carrier disc and in each case sweep over the coating of the carrier disc in its marginal region. Owing to the vertical constraining forces which occur during the guiding of the carrier disc, the coating of the carrier discs is therefore exposed to particularly high wear in the guided region, when a device according to JP 11254303 A2 is employed. Another disadvantage of using the solution proposed in JP 11254303 A2 for PPG methods is therefore that the guide ring is engaged far into the carrier disc and can thus damage the carrier disc coating (for example polyurethane).
No satisfactory solution to the problem of carrier disc bending in the region of the workpiece excursion is therefore known from the prior art.